A prior art modulator, shown in FIG. 1, is described in U.S. Pat. No. 9,664,931, issued May 30, 2017. Another prior modulator, also described in in U.S. Pat. No. 9,664,931, issued May 30, 2017 is shown in FIG. 2. This prior modulator has two drive electrodes and a third electrode that is floating. This prior modulator uses lithium niobate for its electro-optical (EO) material and the lithium niobate material of this prior modulator is a Z-cut material and thus the electrodes need to be located above and below the EO waveguides, rather than at the lateral sides of the EO waveguides.
A prior modulator that makes use of a X-cut lithium niobate material is described by I. L. Gheorma, P. Savi and R. M. Osgood, Jr. in “Thin Layer Design of X-Cut LiNbO3 Modulators,” IEEE Photonics Technology Letters, v. 12, n. 12, December 2000, p. 1618. For this prior modulator, shown in FIG. 3, the metal electrodes are located at the sides of ridges formed of the lithium niobate material containing the two EO waveguides. However, a portion of the lithium niobate material extends laterally beyond those ridges and underneath the metal electrodes. Thus, since the dielectric constant of lithium niobate is very high, a significant portion of the electric field is present in the extended portions of lithium niobate material rather than overlapping with the light propagating in the two waveguides. A similar structure is described by S. J. Chang, et al. in “Improved Electrooptic Modulator with Ridge Structure in X-cut LiNbO3,” Journal of Lightwave Technology, v. 17. N. 5, May 1999, p. 843.
Yet another prior modulator described by A. Rao, P. Rabiei and S. Fathpour in “Heterogeneous microring and Mach-Zehnder modulators based on lithium niobate and chalcogenide glasses on silicon,” Optics Express, v. 23, no. 17, 24 Aug. 2015, p. 22746, makes use of a thin film of Y-cut lithium niobate material with the light propagating along the material X axis. This structure places the metal electrodes above the lithium niobate film, as illustrated in FIG. 4. Again, a substantial portion of the modulating electric field is dropped across the portions of the lithium niobate layer where the light is not present. Thus, the modulation efficiency of this prior structure is poorer. The best value for Vπ (the RF drive voltage needed to achieve full on/off modulation of the output light) achieved with this prior structure is 3.8 volts at a low modulation frequency.
Another prior modulator structure, illustrated in FIG. 5, likewise places the electrodes at the top surface of a thin but laterally continuous layer of lithium niobate. This prior modulator is described by S. Sriram and V. Stenger in Technical Digest of the Conference on Lasers and ElectroOptics CLEO2013, paper CW30.3 (Optical Society of America 2013. A value of 2.5 volts was obtained for Vπ at DC.
Direct integration of an electro-optic modulator with an antenna is described by R. B. Waterhouse and D. Novak in “Integrated antenna/electrooptic modulator for RF photonic front-ends,” Proceedings 2011 International Microwave Symposium, June 2011, IEEE, and uses a conventional Z-cut lithium niobate modulator integrated with a printed patch antenna.
Only the modulators described in U.S. Pat. No. 9,664,931, issued May 30, 2017 have two strips of lithium niobate EO material that are optically and physically separate from each other except at the longitudinal ends of those strips. For all the other prior modulators based on lithium niobate material, additional lithium niobate material located underneath or at the sides of the two EO strips connect those strips together, as shown in FIGS. 3-5. Thus, all of the prior art modulators except the modulators described in U.S. Pat. No. 9,664,931 have both EO waveguides sharing a common piece of EO material that is laterally contiguous, meaning the two EO waveguides are contiguous in the cross section made through the pair of phase-modulation arms of the modulator. According to the publications of Gheorma and of Chang, referenced above, the metal electrodes placed against the shorter sides (or edges) of the optical waveguide abut only a portion of those edges. According to the later devices of Sriram and Stenger, referenced above, the metal electrodes are placed above the lithium niobate layer. According to the even later devices of Rao, Rabiei and Fathpour, referenced above, the metal electrodes are placed above the lithium niobate layer and a dielectric strip also is placed above the lithium niobate layer, on the same side of the lithium niobate layer as the metal electrodes.
None of the prior art modulators that have thin layers of lithium niobate are designed to operate in a push-pull manner with a balanced two-conductor RF feed.
Another electro-optic optical modulator is described by Rabiei in U.S. Pat. No. 9,746,743, issued Aug. 29, 2017, which is incorporated herein by reference.
What is needed is an improved electro-optic modulator that operates in a balanced push-pull manner. The embodiments of the present disclosure answer these and other needs.